The Pipe/Flange set up is pretty sturdy when it is on soft malleable ground like sand, grass, dirt, etc. but it does wiggle a bit on concrete......although I don't how often I'll use the table on concrete I'm not much of a "tailgater".
The downside of the pipe/flange set up is the added weight of steel, but it really isn't that bad. I'm all about the "reuse" idea but if you can't find an old table to salvage the legs, I saw a thread on the Buzz about pre-fab legs from these guys:
:: Ebco Products Corp - Table Legs & Folding Legs ::
I've also seen tables using the legs from Roll-a-table and T-nuts and that seems to work pretty well.
As for using plywood It should work fine but I wouldn't use OSB and probably go with 3/4" if its going to double as a seat. If the Plywood is regular exterior plywood then you will need to waterproof it to prevent warping or you could use marine plywood but I still may coat that too. You can do this with epoxy or spar urethane (like I did), if you go with epoxy then you will need to coat it with a UV protectant because the epoxy is not UV sturdy.
Nevertheless, I'm not much of a woodworker myself except for taking wood shop in high school. But there are plenty of small projects much like mine on the Buzz that can help you with yours, it just takes a little bit of digging and maybe some stupid questions here and there.
I hope this helps and Good Luck!